A method for separating and recovering phosphorus and arsenic from phosphorus-containing arsenic solutions

Abstract

This invention relates to the field of chemical separation technology, specifically to a method for separating and recovering phosphorus and arsenic from a phosphorus- and arsenic-containing solution. The method involves adding sodium aluminate to the phosphorus- and arsenic-containing solution and conducting a hydrothermal reaction. The reaction products are then separated into a solid phase of tetrasodium aluminum hydroxydiphosphate and an arsenic-containing liquid phase. A calcium salt is added to the arsenic-containing liquid phase for a causticization reaction to obtain calcium arsenate precipitate. The pH of the phosphorus- and arsenic-containing solution is 10.0-11.0, the hydrothermal reaction temperature is greater than 150°C, and the holding time is 2-6 hours. This method achieves efficient fractional recovery of phosphorus and arsenic. It is a simple, highly selective, efficient, and low-cost alkaline system phosphorus and arsenic separation method that can simultaneously realize the resource recovery of phosphorus and arsenic, possessing significant environmental and economic value.

Description

Technical Field

[0001] This invention relates to the field of chemical separation technology, and more specifically, to a method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution. Background Technology

[0002] Sodium phosphate and sodium arsenate are both acid salts of a strong base and a weak acid. They have highly similar chemical properties and both exhibit high solubility in alkaline aqueous solutions, making it difficult to achieve highly selective separation using conventional methods. In the production processes of industries such as metallurgy, chemicals, semiconductors, and pesticides, large quantities of alkaline wastewater or process waste liquid containing both sodium phosphate and sodium arsenate are generated. Arsenic is a highly toxic pollutant; direct discharge would cause serious environmental pollution and ecological harm. Furthermore, both phosphorus and arsenic in the solution are valuable recyclable resources, and direct disposal would result in resource waste.

[0003] Current technologies for separating phosphorus and arsenic in alkaline aqueous solutions have the following main drawbacks: (1) Conventional chemical precipitation method: Conventional precipitants such as iron salts, aluminum salts, and calcium salts have extremely poor selectivity for precipitating phosphate and arsenate under alkaline conditions. They often precipitate phosphorus and arsenic simultaneously, making it impossible to effectively separate the two. Moreover, the precipitated products are mixed hazardous wastes, which are difficult and costly to dispose of and cannot be recycled.

[0004] (2) Solvent extraction method: There are very few types of extractants suitable for alkaline systems. Emulsification is prone to occur during the extraction process. The extractant is lost in large quantities, the operating cost is high, and it is easy to generate secondary organic pollution, making it difficult to apply on a large scale in industry.

[0005] (3) Ion exchange method: Ion exchange resins have extremely poor adsorption selectivity for phosphate and arsenate, are easily interfered with by other anions in the solution, have low adsorption capacity, require frequent regeneration, and have extremely low efficiency when treating high concentration solutions, which cannot meet the requirements of continuous industrial operation.

[0006] (4) Crystallization separation method: It requires extremely high solution concentration and strict temperature control. The separation efficiency is low, the energy consumption is high, and the co-crystallization of phosphorus and arsenic is serious, so the separation effect cannot meet expectations.

[0007] In the existing technology, although there are reports on the precipitation of phosphate ions using aluminum salts in acidic systems, this method cannot be effectively implemented in alkaline systems and cannot selectively separate phosphate and arsenate ions. There are also studies on the preparation of hydroxyaluminate phosphate compounds by hydrothermal methods, but they have not been applied to the selective separation of phosphorus and arsenic in alkaline systems, and a complete process for the simultaneous resource recovery of phosphorus and arsenic has not been formed. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for separating and recovering phosphorus and arsenic in a phosphorus-containing arsenic solution.

[0009] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: This invention provides a method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution. Sodium aluminate is added to the phosphorus-containing arsenic solution and a hydrothermal reaction is carried out. The reaction product is separated into a solid phase of tetrasodium aluminum hydroxybisphosphate and an arsenic-containing liquid phase. Calcium salt is added to the arsenic-containing liquid phase and a causticization reaction is carried out to obtain calcium arsenate precipitate. The pH value of the phosphorus-arsenic solution is 10.0-11.0, the temperature of the hydrothermal reaction is greater than 150℃, and the holding time is 2-6 hours.

[0010] Based on the above technical solution, the present invention can be further improved as follows.

[0011] Furthermore, the hydrothermal reaction temperature is 180°C, and the holding time is 4 hours.

[0012] Furthermore, the causticizing reaction is carried out at a temperature of 90℃-100℃ for 1-2 hours.

[0013] Furthermore, the amount of sodium aluminate added is determined according to the ratio of Al element to PO4 in the solution. 3- The molar ratio is 1.0-1.2:1, and the amount of calcium salt added is based on the ratio of Ca element to AsO4 in the solution. 3- The molar ratio is 1.5-3.0:1.

[0014] Furthermore, the calcium salt is one or more of calcium oxide, calcium hydroxide, and calcium chloride.

[0015] Furthermore, the phosphorus-arsenic-containing solution contains sodium phosphate and sodium arsenate, with the concentration of sodium phosphate being 5-70 g / L and the concentration of sodium arsenate being 5-70 g / L.

[0016] Furthermore, the following steps are included: S1. Obtain the solution to be treated and measure the initial pH value. Adjust the pH value of the solution to be treated according to the initial pH value to obtain the phosphorus-arsenic-containing solution. S2. Sodium aluminate is mixed with the solution obtained in step S1 and stirred for 10-30 minutes. Then it is added to a closed hydrothermal reaction device to carry out the hydrothermal reaction at a pressure of 0.9-1.0 MPa. After the reaction is completed, it is naturally cooled to room temperature. The solid-liquid separation is carried out by vacuum filtration to obtain a filtrate containing sodium arsenate and a filter cake containing tetrasodium aluminum hydroxybisphosphate. S3. Add calcium salt to the filtrate containing sodium arsenate, stir and carry out the causticization reaction, and filter the solution after the reaction to obtain calcium arsenate filter cake and causticization reaction filtrate.

[0017] Furthermore, in step S1, the pH adjustment method is as follows: When the initial pH value is greater than 11.0, the pH value is neutralized to 10.0-11.0 using an acidic solution; When the initial pH value is less than 10.0, an alkaline solution is used to neutralize the pH value to 10.0-11.0; When the initial pH is 10.0-11.0, proceed directly to step S2.

[0018] Furthermore, it also includes an alkali recovery step, wherein the alkali recovery method is to evaporate the causticization reaction filtrate from step S3 to obtain liquid alkali.

[0019] Furthermore, when the phosphorus-arsenic solution contains sodium sulfate, before completing step S2 and proceeding to step S3, the solution also includes a step of cooling and crystallizing the sodium arsenate-containing filtrate at 0-5°C; after the cooling and crystallization is completed, sodium sulfate decahydrate crystals precipitate out of the filtrate, and after separating the sodium sulfate decahydrate crystals, the filtrate is then treated in step S3.

[0020] The present invention provides a method for separating and recovering phosphorus and arsenic in phosphorus- and arsenic-containing solutions. By precisely controlling the pH value of the solution within the range of 10.0-11.0 and using a hydrothermal reaction temperature above 150°C combined with a holding time of 2-6 hours, the method achieves the selective reaction of phosphate ions with sodium aluminate to form stable tetrasodium aluminum hydroxydiphosphate crystals. Under these conditions, arsenate ions cannot form stable precipitates with sodium aluminate and remain completely in the solution. As a result, the phosphorus precipitation rate reaches over 99% and the arsenic loss rate is less than 1%. This method completely solves the industry pain point of phosphorus and arsenic being difficult to separate due to their similar chemical properties in alkaline systems and has extremely high separation selectivity.

[0021] The beneficial effects of this invention are as follows: (1) The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solution of the present invention can achieve efficient separation and simultaneous recovery of phosphorus and arsenic through only two core processes: hydrothermal selective precipitation and calcium salt precipitation, with good separation effect; (2) The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solution of the present invention uses readily available sodium aluminate and calcium salts, does not produce harmful substances in the reaction process, and the products can be recovered without secondary pollution, which meets the requirements of green chemical development. (3) The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solution of the present invention has high purity of the separated tetrasodium aluminum hydroxydiphosphate, which can be directly used for high-value utilization. (4) The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solution of the present invention can obtain calcium arsenate, which can be recycled as arsenic resource raw material or safely solidified, greatly reducing the processing cost and having significant environmental and economic benefits. (5) The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solution of the present invention has mild and controllable process conditions, is easy to operate, can be directly connected to existing industrial production lines, and is easy to scale up and apply. Attached Figure Description

[0022] Figure 1 A schematic diagram illustrating the steps of the method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to the present invention; Figure 2 The present invention provides a detailed flowchart of the method for separating and recovering phosphorus and arsenic from phosphorus-containing arsenic solutions. Figure 3 The X-ray diffraction pattern of the tetrasodium aluminum hydroxybisphosphate filter cake in Example 1 of the present invention is shown. Detailed Implementation

[0023] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0024] The present invention discloses a method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution. Sodium aluminate is added to the phosphorus-containing arsenic solution and a hydrothermal reaction is carried out. The reaction product is then subjected to solid-liquid separation to obtain a tetrasodium aluminum hydroxybisphosphate solid phase and an arsenic-containing liquid phase. Calcium salt is added to the arsenic-containing liquid phase to carry out a causticization reaction to obtain calcium arsenate precipitate. The pH value of the phosphorus-containing arsenic solution is 10.0-11.0, the hydrothermal reaction temperature is greater than 150°C, and the holding time is 2-6 hours.

[0025] The present invention provides a method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solutions. This method achieves graded recovery of phosphorus and arsenic. Phosphate ions react selectively with sodium aluminate to form stable tetrasodium aluminum hydroxybisphosphate crystals, while arsenate ions cannot form stable precipitates with sodium aluminate under these conditions and remain completely in the solution. This results in a phosphorus precipitation rate of over 99% and an arsenic loss rate of less than 1%, completely solving the technical problem of difficulty in separating phosphorus and arsenic due to their similar chemical properties in alkaline systems. This method exhibits extremely high separation selectivity.

[0026] The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solutions of the present invention can achieve efficient separation and simultaneous recovery of phosphorus and arsenic through only two core processes: hydrothermal selective precipitation and calcium salt precipitation. It does not require complex extraction, ion exchange and other processes and equipment. The process conditions are mild and controllable, the operation is convenient, and it can be directly connected to existing industrial production lines, making it easy to scale up and apply.

[0027] The present invention discloses a method for separating and recovering phosphorus and arsenic from phosphorus-containing arsenic solutions. The separated tetrasodium aluminum hydroxydiphosphate is of high purity and can be directly sold as a flame retardant, ceramic additive, or used as a high-value raw material in the phosphorus chemical industry. Calcium arsenate can be recycled as an arsenic resource or safely solidified. The causticized filtrate can also be treated to achieve alkali resource recovery. Overall, this method achieves hazardous waste reduction and resource recycling, significantly reducing treatment costs and providing both significant environmental and economic benefits. Furthermore, the sodium aluminate and calcium salts used are conventional industrial bulk raw materials, widely available and inexpensive. The reaction process is carried out in a closed system without generating harmful gases, and the treated effluent can be recycled back to the production process, resulting in no secondary pollution and meeting the requirements of green chemical development.

[0028] Preferably, the hydrothermal reaction temperature is 180℃ and the holding time is 4 hours.

[0029] Preferably, the causticizing reaction is carried out at a temperature of 90℃-100℃ for 1-2 hours.

[0030] Preferably, the amount of sodium aluminate added is based on the ratio of Al element to PO4 in the solution. 3- The molar ratio is 1.0-1.2:1, and the amount of calcium salt added is based on the ratio of Ca element to AsO4 in the solution. 3- The molar ratio is 1.5-3.0:1.

[0031] Preferably, the calcium salt is one or a combination of calcium oxide, calcium hydroxide, and calcium chloride.

[0032] Preferably, the phosphorus-arsenic solution contains sodium phosphate and sodium arsenate, with the concentration of sodium phosphate being 5-70 g / L and the concentration of sodium arsenate being 5-70 g / L; in addition, the phosphorus-arsenic solution may also contain other metal salts.

[0033] Specifically, such as Figure 1 and 2 As shown, the method of the present invention includes the following steps: S1. Obtain the solution to be treated and measure the initial pH value. Adjust the pH value of the solution to be treated in the solution conditioning tank according to the initial pH value to obtain a phosphorus and arsenic-containing solution.

[0034] The pH control method is as follows: When the initial pH value is greater than 11.0, an acidic solution is used to neutralize the pH value to 10.0-11.0; the acidic solution can be one or both of dilute sulfuric acid solution and dilute hydrochloric acid solution.

[0035] When the initial pH value is less than 10.0, an alkaline solution is used to neutralize the pH value to 10.0-11.0; the alkaline solution can be one or both of sodium hydroxide solution and potassium hydroxide solution.

[0036] When the initial pH is 10.0-11.0, proceed directly to step S2.

[0037] S2. Mix sodium aluminate with the solution obtained in step S1 and stir for 10-30 minutes to completely dissolve the sodium aluminate. Then add it to a closed hydrothermal reactor for hydrothermal reaction at a pressure of 0.9-1.0 MPa. After the reaction is completed, allow it to cool naturally to room temperature and perform solid-liquid separation to obtain a filtrate containing sodium arsenate and a filter cake containing tetrasodium aluminum hydroxybisphosphate.

[0038] The specific reaction formula for this step is as follows: 2Na3PO4+NaAlO2+2H2O=Na4Al(OH)(PO4)2↓+3NaOH Preferably, the solid-liquid separation method is filter press filtration.

[0039] The tetrasodium aluminum hydroxydiphosphate filter cake obtained in this step can be used as a product or a raw material for phosphorus chemical industry after washing and drying, thus realizing the resource recovery of phosphorus.

[0040] S3. Add calcium salt to the filtrate containing sodium arsenate, stir and carry out the causticization reaction. Filter the solution after the reaction to obtain calcium arsenate filter cake and causticization reaction filtrate. The filtration method can be pressure filtration.

[0041] The reaction formula for this step is as follows: 2AsO4 3- +3Ca 2+ =Ca3(PO4)2↓ The calcium arsenate filter cake obtained in this step can be used as a product or as a raw material for arsenic chemical processing after washing and drying, thus realizing the resource recovery of arsenic.

[0042] Preferably, the method of the present invention further includes an alkali recovery step, wherein the alkali recovery is carried out by evaporating and concentrating the causticizing reaction filtrate in step S3 in an evaporator to obtain liquid alkali, thereby realizing the resource recovery of alkali.

[0043] Preferably, when the phosphorus-arsenic solution contains sodium sulfate, before completing step S2 and proceeding to step S3, the solution further includes a step of cooling and crystallizing the sodium arsenate filtrate at 0-5°C; after cooling and crystallization, sodium sulfate decahydrate crystals precipitate out of the filtrate, and after separating the sodium sulfate decahydrate crystals, the filtrate is then treated in step S3.

[0044] The method for separating and recovering phosphorus and arsenic in phosphorus-containing arsenic solutions of the present invention is a simple, highly selective, efficient, and low-cost alkaline system phosphorus and arsenic separation method that can simultaneously realize the resource utilization of phosphorus and arsenic, and has important environmental and economic significance.

[0045] The effects of the present invention will be illustrated below through specific embodiments and comparative examples.

[0046] Example 1 This embodiment treats an alkaline process solution containing phosphorus and arsenic produced in a metallurgical industry. The P2O5 concentration is 20 g / L, and the As2O3 concentration is 15 g / L, existing as sodium phosphate and sodium arsenate, respectively. The specific steps are as follows: S1. pH Measurement and Control: Take 1L of the above solution for pH measurement. The initial pH value of the solution is 10.5, which meets the treatment requirements. The solution to be treated is a phosphorus and arsenic-containing solution, which can be directly used in step S2 without adjustment.

[0047] S2, Hydrothermal Selective Precipitation of Phosphorus: Sodium aluminate is added to the phosphorus- and arsenic-containing solution from step S1, in an amount equal to Al and PO4. 3- The molar ratio is 0.5:1; stir for 20 minutes until completely dissolved and mixed evenly, then transfer to a closed hydrothermal reactor lined with polytetrafluoroethylene, pressurize at 0.9-1.0 MPa, heat to 180℃ and keep warm for 4 hours; after the reaction is complete, cool naturally to room temperature, and separate the liquid and solid under vacuum to obtain a filtrate containing sodium arsenate and a filter cake containing tetrasodium aluminum hydroxybisphosphate.

[0048] The mass of the tetrasodium aluminum hydroxydiaphosphate filter cake obtained in this example was determined to be 61.83 g, and the mass after thorough washing and drying with hot water was 45.87 g. The P2O5 content was found to be 43.5%, and X-ray diffraction (XRD) analysis showed that its phase was tetrasodium aluminum hydroxydiaphosphate (Na4Al(OH)(PO4)2). In the obtained arsenic-containing filtrate, the P2O5 concentration was 0.02 g / L, the As2O3 concentration was 14.76 g / L, and the sodium hydroxide concentration was 16.88 g / L.

[0049] Calculations show that the phosphorus precipitation rate is 99.9%, indicating excellent separation performance. The arsenic loss rate is 1.61%, mainly due to the phosphorus-containing filter cake as an adjunct. After washing, the arsenic is added to the phosphorus-containing filter cake washing solution for secondary recovery or disposal. The sodium hydroxide produced by the hydrothermal reaction can be recovered as alkali in the later stage.

[0050] S3. Causticization precipitation treatment of arsenic: Add calcium hydroxide to the arsenic-containing filtrate obtained in step S2, with the amount added according to the molar ratio of Ca to As of 2:1. Stir the reaction at 90-100℃ for 2 hours, and filter to obtain calcium arsenate filter cake and causticization reaction filtrate.

[0051] The mass of the calcium arsenate filter residue after solid-liquid separation was determined to be 45.45 g, with a moisture content of 21.8%. After thorough washing with hot water and drying, the mass of the filter residue was 35.16 g. Analysis of the filter cake composition revealed a P2O5 content of 0.06%, an As2O3 content of 41.96%, and a CaO content of 47.43%. Calculations showed that the main components of the causticized residue containing arsenic were calcium arsenate (approximately 84%), calcium hydroxide (excessive reaction, approximately 15%), and trace amounts of calcium phosphate. This residue can be used for arsenic resource recovery or for hazardous waste solidification treatment.

[0052] The causticized filtrate contained 0.002 mg / L of P2O5, 0.02 mg / L of As2O3, and 36.8 g / L of NaOH, with removal rates of phosphorus and arsenic ≥99.99%.

[0053] Causticization filtrate alkali recovery: The filtrate after causticization in step S3 is mainly sodium hydroxide solution with a content of 36.8 g / L and a mass concentration of 3.53%. The low-concentration alkali solution can be concentrated by evaporation to a concentration of ≥30% and sold as liquid alkali product.

[0054] Phosphorus resource utilization: After the filter cake of tetrasodium aluminum hydroxydiphosphate is washed twice with deionized water and dried at 105℃, high-purity tetrasodium aluminum hydroxydiphosphate product is obtained, which can be directly used in the production of flame retardants and ceramic additives.

[0055] Example 2 This embodiment treats a phosphorus- and arsenic-containing alkaline process solution generated in a vanadium metallurgical industry. The solution contains 10.8 g / L of P₂O₅ and 5.6 g / L of As₂O₃, which exist as sodium dihydrogen phosphate and sodium dihydrogen arsenate, respectively; the concentration of Na₂SO₄ is 30.4 g / L. The specific steps are as follows: S1. pH Measurement and Adjustment: Take 1 L of the above solution and measure its initial pH value as 3.5. Adjust the pH to 10.5 using solid sodium hydroxide and stir thoroughly to obtain a phosphorus-arsenic-containing solution.

[0056] S2, Hydrothermal Selective Precipitation of Phosphorus: Sodium aluminate is added to the phosphorus- and arsenic-containing solution from step S1, in an amount equal to Al and PO4. 3- The molar ratio is 0.5:1; stir for 20 minutes until completely dissolved and mixed evenly, then transfer to a closed hydrothermal reactor lined with polytetrafluoroethylene, pressurize at 0.9-1.0 MPa, heat to 180℃ and keep warm for 4 hours; after the reaction is complete, cool naturally to room temperature, and separate the liquid and solid under vacuum to obtain a filtrate containing sodium arsenate and a filter cake containing tetrasodium aluminum hydroxybisphosphate.

[0057] The mass of the tetrasodium aluminum hydroxydiaphosphate filter cake in this embodiment was determined to be 33.6 g, and the mass after thorough washing and drying with hot water was 24.8 g. The P2O5 content was found to be 43.5%, and X-ray diffraction (XRD) analysis confirmed that the phase was tetrasodium aluminum hydroxydiaphosphate (Na4Al(OH)(PO4)2). The arsenic-containing filtrate was analyzed, and the concentrations were: P2O5 0.01 g / L, As2O3 5.61 g / L, sodium hydroxide 9.14 g / L, and sodium sulfate 30.5 g / L.

[0058] Calculations show that the phosphorus precipitation rate is 99.9%, indicating excellent separation performance. The arsenic loss rate is 0.84%, mainly due to the phosphorus-containing filter cake as an adjunct. After washing, the arsenic is added to the phosphorus-containing filter cake washing solution for secondary recovery or disposal. The sodium hydroxide produced by the hydrothermal reaction can be recovered as alkali in the later stage.

[0059] S3. Causticization precipitation treatment of arsenic: Add calcium hydroxide to the arsenic-containing filtrate obtained in step S2, with the amount added according to the molar ratio of Ca to As of 3:1. Stir the reaction at 90-100℃ for 2 hours, and filter to obtain calcium arsenate filter cake and causticization reaction filtrate.

[0060] The mass of the calcium arsenate filter cake after solid-liquid separation was determined to be 27.01 g, with a moisture content of 20.2%. After thorough washing with hot water and drying, the mass of the filter cake was 21.33 g. The composition of the filter cake was analyzed, revealing a P2O5 content of 0.03%, an As2O3 content of 26.04%, a CaO content of 44.12%, and an SO3 content of 23.93%.

[0061] Calculations show that the main components of the causticized residue containing arsenic filtrate are calcium arsenate (approximately 52%), calcium sulfate (approximately 41%), calcium hydroxide (excessive reaction, approximately 7%), and trace amounts of calcium phosphate, which are used for hazardous waste solidification treatment.

[0062] The causticized filtrate contained 0.007 mg / L of P2O5, 0.03 mg / L of As2O3, 21.08 g / L of NaOH, and 21.33 g / L of Na2SO4, with removal rates of phosphorus and arsenic ≥99.99%.

[0063] Causticization filtrate alkali recovery: The filtrate after causticization in step S3 is mainly sodium hydroxide and sodium sulfate solution, with mass concentrations of 2.03% and 2.05%, respectively. The filtrate is separated by nanofiltration membrane to separate dilute sodium hydroxide solution, which is further concentrated to achieve resource recovery as liquid alkali. The membrane concentrate is sodium sulfate and a small amount of sodium hydroxide. Sodium sulfate (anhydrous sodium sulfate or sodium sulfate decahydrate) is first recovered by evaporation or cooling crystallization, and then sodium hydroxide is further recovered.

[0064] Phosphorus resource utilization: After the filter cake of tetrasodium aluminum hydroxydiphosphate is washed twice with deionized water and dried at 105℃, high-purity tetrasodium aluminum hydroxydiphosphate product is obtained, which can be directly used in the production of flame retardants and ceramic additives.

[0065] Example 3 This embodiment describes an alkaline process solution produced in a metallurgical industry, containing sodium phosphate, sodium arsenate, and sodium carbonate. The concentration of sodium phosphate (as P2O5) is 38 g / L, the concentration of sodium arsenate (as As2O3) is 35 g / L, and the concentration of sodium carbonate is 13 g / L. The specific steps are as follows: S1. pH Measurement and Control: Take 1L of the above solution and measure its initial pH value as 14. Adjust the pH to 10.5 using 98% concentrated sulfuric acid. During the pH adjustment process, sodium carbonate reacts with concentrated sulfuric acid to produce sodium sulfate, carbon dioxide, and water. Stir until the pH stabilizes at 10-10.5 to obtain a phosphorus-arsenic-containing solution.

[0066] S2, Hydrothermal Selective Precipitation of Phosphorus: Sodium aluminate is added to the phosphorus- and arsenic-containing solution from step S1, in an amount equal to Al and PO4. 3- The molar ratio is 0.5:1; stir for 20 minutes until completely dissolved and mixed evenly, then transfer to a closed hydrothermal reactor lined with polytetrafluoroethylene, pressurize at 0.9-1.0 MPa, heat to 180℃ and keep warm for 4 hours; after the reaction is complete, cool naturally to room temperature, and separate the liquid and solid under vacuum to obtain a filtrate containing sodium arsenate and a filter cake containing tetrasodium aluminum hydroxybisphosphate.

[0067] The mass of the tetrasodium aluminum hydroxydiaphosphate filter cake was determined to be 116.96 g, which decreased to 87.2 g after thorough washing with hot water and drying. The P2O5 content was found to be 43.6%, and X-ray diffraction (XRD) analysis confirmed its phase as tetrasodium aluminum hydroxydiaphosphate (Na4Al(OH)(PO4)2). In the arsenic-containing filtrate, the P2O5 concentration was 0.02 g / L, the As2O3 concentration was 36.9 g / L, the sodium hydroxide concentration was 33.86 g / L, and the sodium sulfate concentration was 18.37 g / L. The calculated phosphorus precipitation rate was 99.9%, indicating excellent separation. The arsenic loss rate was 2.8%, mainly due to the phosphorus-containing filter cake as an adjunct. After washing, the arsenic was transferred to the phosphorus-containing filter cake washing solution for secondary recovery or disposal. The sodium hydroxide produced by the hydrothermal reaction can be recovered as alkali later.

[0068] S3. Causticization precipitation treatment of arsenic: Add calcium hydroxide to the arsenic-containing filtrate obtained in step S2, with the amount added according to the molar ratio of Ca to As of 2:1. Stir the reaction at 90-100℃ for 2 hours, and filter to obtain calcium arsenate filter cake and causticization reaction filtrate.

[0069] The mass of the calcium arsenate filter cake after solid-liquid separation was determined to be 104g, with a moisture content of 18.2%. After thorough washing with hot water and drying, the mass of the filter cake was 83.3g. The composition of the filter cake was analyzed, showing a P2O5 content of 0.02%, an As2O3 content of 40.84%, a CaO content of 46.18%, and an SO3 content of 3.43%.

[0070] Calculations show that the main components of the causticized residue containing arsenic filtrate are calcium arsenate (approximately 82%), calcium sulfate (approximately 6%), calcium hydroxide (excessive reaction, approximately 12%), and trace amounts of calcium phosphate, which are used for hazardous waste solidification treatment.

[0071] The causticized filtrate contained 0.001 mg / L of P2O5, 0.5 mg / L of As2O3, 21.08 g / L of NaOH, and 21.33 g / L of Na2SO4, with removal rates of phosphorus and arsenic ≥99.99%.

[0072] Causticization filtrate alkali recovery: The filtrate after causticization in step S3 is mainly sodium hydroxide and sodium sulfate solution, with mass concentrations of 7.54% and 1.19%, respectively. The filtrate is separated by nanofiltration membrane to separate dilute sodium hydroxide solution, which is further concentrated to achieve resource recovery as liquid alkali. The membrane concentrate is sodium sulfate and a small amount of sodium hydroxide. Sodium sulfate (anhydrous sodium sulfate or sodium sulfate decahydrate) is first recovered by evaporation or cooling crystallization, and then sodium hydroxide is further recovered.

[0073] Phosphorus resource utilization: After the filter cake of tetrasodium aluminum hydroxydiphosphate is washed twice with deionized water and dried at 105℃, high-purity tetrasodium aluminum hydroxydiphosphate product is obtained, which can be directly used in the production of flame retardants and ceramic additives.

[0074] Example 4 This embodiment describes an alkaline process solution produced in a metallurgical industry, containing sodium hydrogen phosphate, sodium hydrogen arsenate, and sodium sulfate. The concentration of sodium phosphate (as P₂O₅) is 15 g / L, the concentration of sodium hydrogen arsenate (as As₂O₃) is 20 g / L, and the concentration of sodium sulfate is 180 g / L. The specific steps are as follows: S1. pH Measurement and Control: Take 1L of the above solution, measure its initial pH value as 9, adjust the pH to 10.5 using solid sodium hydroxide, stir evenly, and obtain the solution to be treated. S2, Hydrothermal Selective Precipitation of Phosphorus: Sodium aluminate is added to the solution to be treated in step S1, with the addition amount according to the ratio of Al to PO4. 3- The molar ratio is 0.5:1; stir for 20 minutes until completely dissolved and mixed evenly, then transfer to a closed hydrothermal reactor lined with polytetrafluoroethylene, pressurize at 0.9-1.0 MPa, heat to 180℃ and keep warm for 4 hours; after the reaction is complete, cool naturally to room temperature, and separate the liquid and solid under vacuum to obtain a filtrate containing sodium arsenate and a filter cake containing tetrasodium aluminum hydroxybisphosphate.

[0075] The mass of the tetrasodium aluminum hydroxydiaphosphate filter cake was determined to be 50.2 g, which decreased to 34.4 g after thorough washing with hot water and drying. The P2O5 content was 43.8%, and X-ray diffraction (XRD) analysis confirmed its phase as tetrasodium aluminum hydroxydiaphosphate (Na4Al(OH)(PO4)2). In the arsenic-containing filtrate, the P2O5 concentration was 0.02 g / L, the As2O3 concentration was 20.16 g / L, the sodium hydroxide concentration was 12.77 g / L, and the sodium sulfate concentration was 181.45 g / L. The calculated phosphorus precipitation rate was 99.9%, indicating excellent separation. The arsenic loss rate was 1.4%, mainly due to the phosphorus-containing filter cake as an adjunct. After washing, the arsenic is transferred to the phosphorus-containing filter cake washing solution for secondary recovery or disposal. The sodium hydroxide produced by the hydrothermal reaction can be recovered as alkali later.

[0076] S3. Causticization precipitation treatment of arsenic: Add calcium hydroxide to the arsenic-containing filtrate obtained in step S2, with the amount added according to the molar ratio of Ca to As of 3:1. Stir the reaction at 90-100℃ for 2 hours, and filter to obtain calcium arsenate filter cake and causticization reaction filtrate.

[0077] The mass of the calcium arsenate filter cake after solid-liquid separation was determined to be 103g, with a moisture content of 19.5%. After thorough washing with hot water and drying, the mass of the filter cake was 77.3g. The composition of the filter cake was analyzed: P2O5 content was 0.01%, As2O3 content was 19.75%, CaO content was 33.51%, and SO3 content was 20.03%. Calculations show that the main components of the causticized residue containing arsenic filtrate are calcium arsenate (approximately 51%), calcium sulfate (approximately 44%), calcium hydroxide (excessive reaction, approximately 5%), and trace amounts of calcium phosphate, which are used for hazardous waste solidification treatment.

[0078] The causticized filtrate contained 0.007 mg / L of P2O5, 0.2 mg / L of As2O3, 53.74 g / L of NaOH, and 135.25 g / L of Na2SO4, with phosphorus and arsenic removal rates ≥99.99%.

[0079] Alkali recovery from causticized filtrate: The filtrate after causticization in step S3 is mainly composed of sodium hydroxide and sodium sulfate solutions with mass concentrations of 5.17% and 13%, respectively. Sodium sulfate (anhydrous sodium sulfate or sodium sulfate decahydrate) is first recovered by evaporation or cooling and crystallization, and then sodium hydroxide is further recovered.

[0080] Phosphorus resource utilization: After the filter cake of tetrasodium aluminum hydroxydiphosphate is washed twice with deionized water and dried at 105℃, high-purity tetrasodium aluminum hydroxydiphosphate product is obtained, which can be directly used in the production of flame retardants and ceramic additives.

[0081] Example 5 This embodiment examines the process of removing sodium sulfate before precipitating arsenic.

[0082] The difference between this embodiment and Embodiment 4 is that, in step S3, before the arsenic precipitation reaction, a portion of sodium sulfate decahydrate is precipitated by cooling crystallization. The solution containing the precipitated sodium sulfate decahydrate is then used in step S3. The causticization reaction operation method of the arsenic filtrate in step S3 of this embodiment is the same as step 3 in Embodiment 4.

[0083] Theoretically, the temperature for cooling crystallization should be in the range of 0-5℃, but in this embodiment, it is controlled at 2℃-3℃. In this embodiment, 285g of sodium sulfate decahydrate was crystallized out. After washing with a saturated sodium sulfate solution, dissolving in deionized water, and then evaporating and crystallizing, an anhydrous sodium sulfate product with a purity ≥99% was obtained.

[0084] After processing, the mass of the solid-liquid separated calcium arsenate filter cake obtained in this embodiment was 88.6g, with a moisture content of 19.2%. After thorough washing with hot water and drying, the mass of the filter cake was 66.4g. The composition of the filter cake was analyzed, showing a P2O5 content of 0.02%, an As2O3 content of 29.75%, a CaO content of 50.46%, and an SO3 content of 8.86%.

[0085] Calculations show that the main components of the causticized residue containing arsenic filtrate are calcium arsenate (approximately 60%), calcium sulfate (approximately 15%), calcium hydroxide (excessive reaction, approximately 25%), and trace amounts of calcium phosphate, which are used for hazardous waste solidification treatment.

[0086] The causticizing filtrate contained 0.008 mg / L of P2O5, 0.22 mg / L of As2O3, 53.73 g / L of NaOH, and 52.98 g / L of Na2SO4.

[0087] As can be seen, compared with Example 4, the amount of slag in this example is reduced by 14%, which is beneficial to energy conservation and emission reduction.

[0088] Comparative Example 1 This comparative example examines the reaction effect under normal pressure. The only difference between this comparative example and Example 1 is that the hydrothermal reaction is not carried out in step S2, but the reaction is carried out at 100°C for 4 hours under normal pressure.

[0089] Experimental results show that no crystals are formed when reacting at 100℃ under normal pressure, indicating that the synthesis of tetrasodium aluminum hydroxydiphosphate requires hydrothermal reaction conditions.

[0090] Comparative Example 2 This comparative study investigates the effect of temperature on the hydrothermal reaction.

[0091] The only difference between this comparative example and Example 1 is that the hydrothermal reaction temperature in step S2 is controlled at 150°C.

[0092] The experimental results of this comparative example are as follows: The mass of the filter cake obtained in step S2 was 19.12 g, and after thorough washing and drying with hot water, it was 14 g. The P2O5 content was detected to be 43.5%, and X-ray diffraction (XRD) analysis showed that its phase was tetrasodium aluminum hydroxydiphosphate (Na4Al(OH)(PO4)2). After solid-liquid separation, the P2O5 concentration in the filtrate was 13.83 g / L, and the calculated phosphorus precipitation rate was 30.5%.

[0093] Compared with Example 1, the phosphorus resource recovery rate of this comparative example is relatively low, which will lead to problems such as large slag volume in subsequent processing.

[0094] Comparative Example 3 This comparative study investigates the effects of temperature and reaction time on the reaction outcome in a hydrothermal reaction.

[0095] The only difference between this comparative example and Example 1 is that the hydrothermal reaction temperature in step S2 is controlled at 150°C and the reaction time is extended to 8 hours.

[0096] The experimental results of this comparative example are as follows: The mass of the filter cake obtained in step S2 was 23.68 g, and after thorough washing and drying with hot water, it was 17.4 g. The P2O5 content was detected to be 43.5%, and X-ray diffraction (XRD) analysis showed that its phase was tetrasodium aluminum hydroxydiphosphate (Na4Al(OH)(PO4)2). In the filtrate after solid-liquid separation, the P2O5 concentration was 12.36 g / L, and the calculated phosphorus precipitation rate was 37.8%.

[0097] Compared with Comparative Example 2, doubling the reaction time only increased the phosphorus precipitation rate by 6.3%, which could not achieve the desired effect of Example 1. Moreover, extending the reaction time would lead to increased energy consumption and decreased production efficiency.

[0098] Comparative Example 4 This comparative study investigated the effect of fractional arsenic precipitation on the reaction results.

[0099] The only difference between this comparative example and Example 3 is that the calcium hydroxide in step S3 is added in two parts. The total amount added is based on a Ca to As molar ratio of 2:1. The first addition is based on a Ca to As molar ratio of 1:1, and the second addition is based on the remaining amount.

[0100] The experimental results of this comparative example are as follows: the mass of the calcium arsenate filter residue after the first causticization was 57g, with a moisture content of 18.2%. After thorough washing with hot water and drying, the mass of the filter residue was 45.6g. The composition of the filter cake was analyzed, with P2O5 content of 0.04%, As2O3 content of 49.70%, and CaO content of 42.22%. Calculations show that the main components of the causticized residue containing arsenic are calcium arsenate (approximately 99.9%) and trace amounts of calcium phosphate, which can be used as a resource for the recovery of arsenic-containing raw materials.

[0101] The mass of the calcium arsenate filter residue after the second causticization was 47.8g, with a moisture content of 19.5%. After thorough washing with hot water and drying, the mass of the filter residue was 37.6g. Analysis of the filter cake composition showed that the P2O5 content was less than 0.001%, the As2O3 content was 29.89%, the CaO content was 51.16%, and the SO3 content was 7.52%. Calculations indicate that the main components of the causticized residue containing arsenic-containing filtrate are calcium arsenate (approximately 60%), calcium sulfate (approximately 13%), calcium hydroxide (excess reacted, approximately 27%), and trace amounts of calcium phosphate. This residue is intended for hazardous waste solidification treatment.

[0102] The causticized filtrate contained 0.02 mg / L of P2O5, 0.12 mg / L of As2O3, 53.74 g / L of NaOH, and 135.25 g / L of Na2SO4, with phosphorus and arsenic removal rates ≥99.99%.

[0103] It can be seen that the results of this comparative example are comparable to those of Example 3, but Example 3 only requires one causticization process, which is simple and efficient.

[0104] Comparative Example 5 This comparative study investigated the effects of the amount of arsenic added during fractional precipitation and the amount of lime added on the reaction results.

[0105] The only difference between this comparative example and Comparative Example 4 is that the amount of calcium hydroxide added is based on a total addition amount of 1.5:1 Ca to As molar ratio, and it is added in two parts. The first addition is based on a 1:1 Ca to As molar ratio, and the second addition is the remaining amount.

[0106] The mass of the calcium arsenate filter residue after the second causticization was 44.46 g, with a moisture content of 20.8%. After thorough washing with hot water and drying, the mass of the filter residue was 34.31 g. Analysis of the filter cake composition showed that the P2O5 content was less than 0.001%, the As2O3 content was 42.76%, the CaO content was 42.07%, and the SO3 content was 8.27%. Calculations indicate that the main components of the causticized residue containing arsenic-containing filtrate are calcium arsenate (approximately 86%) and calcium sulfate (approximately 14%), and it is intended for hazardous waste solidification treatment.

[0107] The causticized filtrate contained 0.02 mg / L of P2O5 and 2.33 g / L of As2O3, with an arsenic removal rate of 93.34%.

[0108] Compared with Comparative Example 4, the amount of secondary causticizing slag was reduced, but the arsenic removal rate was lower than that of Comparative Example 4.

[0109] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution, characterized in that, Sodium aluminate was added to a phosphorus-arsenic solution and a hydrothermal reaction was carried out. The reaction product was separated into solid and liquid phases to obtain a tetrasodium aluminum hydroxybisphosphate solid phase and an arsenic-containing liquid phase. Calcium salt was added to the arsenic-containing liquid phase to carry out a causticization reaction to obtain calcium arsenate precipitate. The pH value of the phosphorus-arsenic solution is 10.0-11.0, the temperature of the hydrothermal reaction is greater than 150℃, and the holding time is 2-6 hours.

2. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 1, characterized in that, The hydrothermal reaction was carried out at a temperature of 180°C for 4 hours.

3. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 1, characterized in that, The causticizing reaction is carried out at a temperature of 90℃-100℃ for 1-2 hours.

4. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 1, characterized in that, The amount of sodium aluminate added is based on the ratio of Al element to PO4 in the solution. 3- The molar ratio is 1.0-1.2:1, and the amount of calcium salt added is based on the ratio of Ca element to AsO4 in the solution. 3- The molar ratio is 1.5-3.0:

1.

5. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 1, characterized in that, The calcium salt is one or more of calcium oxide, calcium hydroxide, and calcium chloride.

6. A method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to any one of claims 1-5, characterized in that, The phosphorus-arsenic solution contains sodium phosphate and sodium arsenate, with the concentration of sodium phosphate being 5-70 g / L and the concentration of sodium arsenate being 5-70 g / L.

7. A method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Obtain the solution to be treated and measure the initial pH value. Adjust the pH value of the solution to be treated according to the initial pH value to obtain the phosphorus-arsenic-containing solution. S2. Sodium aluminate is mixed with the phosphorus-arsenic-containing solution and stirred before being added to a closed hydrothermal reactor for the hydrothermal reaction at a pressure of 0.9-1.0 MPa. After the reaction is completed, the mixture is naturally cooled to room temperature and filtered to obtain sodium arsenate filtrate and tetrasodium aluminum hydroxybisphosphate filter cake. S3. Add calcium salt to the sodium arsenate filtrate, stir and carry out the causticization reaction, and filter the solution after the reaction to obtain calcium arsenate filter cake and causticization reaction filtrate.

8. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 7, characterized in that, In step S1, the pH adjustment method is as follows: When the initial pH value is greater than 11.0, the pH value is neutralized to 10.0-11.0 using an acidic solution; When the initial pH value is less than 10.0, an alkaline solution is used to neutralize the pH value to 10.0-11.0; When the initial pH is 10.0-11.0, proceed directly to step S2.

9. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 7, characterized in that, It also includes an alkali recovery step, wherein the alkali recovery method is to evaporate the causticization reaction filtrate in step S3 to obtain liquid alkali.

10. The method for separating and recovering phosphorus and arsenic from a phosphorus-containing arsenic solution according to claim 7, characterized in that, When the phosphorus-arsenic solution contains sodium sulfate, before completing step S2 and proceeding to step S3, the solution also includes a step of cooling and crystallizing the sodium arsenate-containing filtrate at 0-5°C; after the cooling and crystallization is completed, sodium sulfate decahydrate crystals precipitate out of the filtrate, and after separating the sodium sulfate decahydrate crystals, the filtrate is then processed in step S3.

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